1. Field of Invention
The present invention relates to a method and apparatus for the indirect determination of molecular weight of gases and their mixtures flowing through the axial or centrifugal compressor based on usage of compressor dynamics. This method is applicable to process controls, compressor controls, and real time analytical data acquisition, where the compressor is used for gas processing.
2. Background
A compressor is a type of rotating equipment commonly used for transfering energy to a flowing gas. The relationship between energy added to the gas, or head, and gas actual volumetric flow represents a measure of a compressor""s performance. Every compressor manufacturer provides a tested map for head and flow parameters as part of the compressor""s specification. Both parameters are multivariable functions of the initial measurable variables, such as pressure and temperature. The performance map reflects a relationship between head and flow by plotting those calculated variables and holding all other non-measurable parameters of the thermodynamic process constant.
A molecular weight measurement is very important to many chemical processes as well as for compressor control and surge protection. The industry uses special chromatography equipment or gas analyzers for these purposes. Most of these methods are very costly and have a significant time delay of output data, which reduces their efficiency and usefulness for process control.
An object of the present invention comprises providing a method of determining the molecular weight of a compressor gas by comparing the ratio of imaginary compressor map conditions and actual compressor operating conditions.
These and other objects of the present invention will become apparent to those skilled in the art upon reference to the following specification, drawings, and claims.
The present invention intends to overcome the difficulties encountered heretofore. To that end, a compressor for processing a compressor gas is provided. A performance map is provided that comprises a plurality of performance curves showing head and flow conditions at various compressor speeds. The performance curves terminate at one end at surge points. Also provided is a processing and calculating unit interfaced with the compressor, for measuring performance parameters of the compressor necessary to determine actual head and flow conditions of the compressor at a given processor speed and for performing calculations. The compressor map performance curves are converted into deviation curves, where the deviation curves represent deviations from the surge point at the head and flow conditions. The deviation curves are merged into a universal compressor curve with a merge-function to within a pre-defined tolerance. Imaginary map head and flow values are calculated using the universal compressor curve, merge-function, deviation curves, and performance curves at the compressor speed. A ratio of the imaginary map head and flow values is compared to a ratio of the actual head and flow values at the compressor speed to determine if the ratios are within a pre-defined tolerance of each other. If the ratios are not within the tolerance, then the imaginary head and flow map values are recalculated by varying either said imaginary head or flow parameters and the comparison is repeated until the ratios are within the pre-defined tolerance. The molecular weight of the compressor gas is calculated using a function of the actual and imaginary flow values and/or head values.